If a customer has the Forecast module and wants to be able to have a Sales Order in the system to create shipments against, but they do not want any of the SO schedules driving MRP. 

In this situation we would suggest that the user leaves the Schedule information Blank and create the qty's needed through the Forecast module.  Forecast will drive MRP and users will be able to still ship them against any sales order in the system.  View the attached Print Screens for further detail.